Permanent magnets (PMs) are used in many applications, in particular as the source of magnetomotive force in motors and generators and as the source of magnetic flux in magnetic bearings. In high-temperature superconducting (HTS) bearings, the rotor of the bearing is typically a PM structure and the stator is an array of HTS elements. PMs used in rotating machinery may utilize either metal or composite overwrap to increase their design limit for high speed rotation, but even with this design the ultimate performance of motors and generators may be limited.
In the case of the HTS bearing, the mechanical strength of the PM may limit the maximum speed at which the bearing can operate. PM materials with the highest energy products and highest available flux, e.g., FeBNd materials, tend to be sintered ceramics that are brittle and have tensile strengths of the order of 10,000 psi. It is possible to increase the maximum rotational speed of the PM rotor by banding the PM on the outside of its circumference either with a high-strength steel material or with a fiber composite material, such as graphite fiber in an epoxy matrix. While such a banding increases the maximum speed, the speed may still be limited.
One of the disadvantages of any permanent magnet structure in which strengthening material is substituted for the magnet material is that as the volume fraction of magnet material decreases, so does the energy product and the available flux. This may decrease the load-carrying capacity of an HTS bearing and the power rating of a motor or generator.
Therefore, magnetic composite structures with high mechanical strength in which magnetic powder is intimately mixed into composite structures to provide permanent magnets of high mechanical strength are needed.